SPM Form 5 Physics - Electronics

CHAPTER 4 ELECTRONICS

4.1 CATHODE RAY OSCILLOSCOPE

What is thermionic emission?• The process of emission of charged particles from a

heated metal surface.

The charged particles are normally electrons.

Thermo (temperature) + Ion (charged carrier)

Factors affecting the rate of emission:• Rate of emission: number of electrons emitted in 1 second• 4 factors:

• Temperature of the heated metal• The higher the temperature, the higher the emission rate.

• Surface area of the heated metal• The larger the surface area of the heated metal, the higher the emission

rate.• Types of metal

• The rate of thermionic emission differs in different types of metal.• Coated material on the metal surface

• Example: The rate of emission will increase when the metal surface is coated by a layer of barium oxide or strontium oxide.

How does thermionic emission occur?

1. Metal consists of a large number of electrons.

2. These electrons are free to move but cannot leave the metal surface because they are held back by attractive forces of the atomic nucleus.

3. When the metal is heated to a certain temperature, some of the electrons gain sufficient energy to escape from the metal to become free electrons.

Thermionic Diode• Diode: An electrical component that only allows current

flow in one direction.• Electrons are only released from the tungsten filament

(when it is hot) and move toward the anode which is connected to the positive terminal.

Electron GunUsed to produce a narrow beam of electrons.

The electrons released by the cathode are accelerated by the accelerating anode and form a narrow beam of electrons.

The beam produced is called a cathode ray.

Properties of cathode ray• Maltese Cross Tube & Deflection Tube

Maltese cross tube Deflection tube

Properties of cathode ray 1. Negatively charged particles called electrons.2. Travel in straight lines and cast sharp shadows.3. Travel at high speed with high kinetic energy.4. Can cause fluorescence. (Kinetic energy of electrons -

light energy) Example: television, computer.5. Can be deflected by electric and magnetic field.

Cathode ray oscilloscope (CRO)• Three components:

• Electron gun• Deflecting plates• Fluorescent screen

Electron GunParts FunctionFilament To heat the cathode.

Cathode Release electrons when heated by the filament.

Control Grid** Grid is connected to a negative potential. The more negative the potential, the more electrons will be repelled from the grid, thus fewer electrons will reach the anode and the screen.The number of electrons reaching the screen determines the brightness of the light.Therefore, the negative potential of the grid can be used as a brightness control.

Focusing and accelerating anode Anode at the positive terminal accelerates the electrons and the electrons are focused into a fine beam as they passed through the anode.

Deflecting platesParts FunctionY-plate Cause deflection in the vertical

direction when a voltage is applied across them.

X-plate Cause deflection in horizontal direction when a voltage is applied across them.

Fluorescent screen1. Screen is coated with a fluorescent salt. Eg: zinc sulphide.2. When the electrons hit the screen, the fluorescent salt will produce a flash of light and hence, a bright spot on the screen.

Function1. Power switch To switch on & turn off the oscilloscope.

2. Focus control To control the focus of the spot on the screen.

3. Intensity control To control brightness of the spot on the screen.

4. X-offset5. Y-offset

Moves the whole trace side to side on screen.Moves the whole trace up and down on screen.

6. Time based control (time/div)

A measure of time for the oscilloscope. When the time-based control, a sawtooth voltage appeared on the X-plates. The electron beam sweep across the screen at a constant speed. By knowing the period of each cycle, T, we can know how fast the beam is sweeping across the screen.

7. Y gain control (volts/div) The “Volts/Div.” wheels amplify an input signal so that for a division a given voltage level is in valid. A “division” is a segment, a square on the screen of the oscilloscope.

Function8. DC/AC switch d.c. – d.c. and a.c. voltage displayed

a.c. – only a.c. voltage displayed9. X-input and Y-input Electric input connected to X-plate and

Y-plate.

Uses of cathode ray oscilloscope

Measuring potential difference• DC voltage• = Displacement of the bright spot from zero position x

selected range of the Y-gain control

What is the value of the dc voltage in both of the figure if the Y-gain control is 2V/division?

Measuring potential difference• AC voltage• = Height of the vertical trace from zero position x selected

range of Y-gain control

Y-gain = 2V/divWhat is the peak ac voltage?

Measure short time intervals

Example:t = 6 x 2 ms = 12 ms = 0.12 s

Try:Time based: 10ms/divTime taken = ?

Time based: 15ms/divTime taken = ?

Displaying wave forms

Problem solving based on CRO display

4.2 SEMICONDUCTORS

Semiconductors• A class of crystalline solid with conductivity between a

conductor and an insulator.• Examples:

• Silicon• Germanium• Boron• Tellurium• Selenium

• Electrical conductivity in semiconductors occurs because there are two types of charged carriers:• Free electrons (negatively charged)• Holes (positively charged)

Describe semiconductors in terms of resistance

Metal InsulatorGood conductors of electricity because they have free electrons that can move easily between atomsResistance of metal is generally very low.

Poor conductors of electricity because they have too few free electrons to move about.Resistance of insulators is very high.

SemiconductorsElectrical conductivity is between that of a conductor and an insulator.Resistance is between that of a conductor and an insulator.At 0 Kelvin, it behaves as an insulator. When temperature increases, the conductivity of electricity will increase because its resistance will be lowered.

Silicon crystal

Silicon has 4 valence electrons.

Each of these 4 electrons are shared with another 4 silicon atoms to form 4 pairs of covalent bonds.

The bonded valence electrons are not free to move. Therefore, silicon is not a good conductor at room temperature.

At room temperature, a silicon crystal acts like an insulator because only a few free electrons and holes are present.

Free electrons and holes• When a bonded electron absorbs heat energy, it is

promoted to a higher energy level.• These electrons are free to move.• A vacancy (hole) is left in the valence shell.• A hole (positively charged) has the tendency to pull

(attract) electrons. • Both the free electrons and the holes help to conduct

electricity.

Resistance change due to temperature change

As temp. increases, more and more electrons are being promoted to higher energy levels and thus, more holes are created.Therefore, the electrical conductivity of a semiconductor increases as the temp. increases. The graph shows the change in

resistance of a conductor and a semiconductor against the change in temp.The resistance of a semiconductor decreases as temp. increases.

When a potential difference is applied to a semiconductor, the electrons and holes will start to flow.Electrons will flow to the negative terminal while the holes will flow to the positive terminal.

Doping• A process of adding small amount of impurities to a

semiconductor to increase its electrical conductivity.• Impurities added are called dopants.• Two types of semiconductor depending on the type od

impurities that are added:• N-type semiconductor• P-type semiconductor

N-type semiconductor• Add pentavalent atoms into a semiconductor.• Pentavalence atoms are atoms that have 5 electrons in

the valence shall.• Example:

• Antimony• Phosphorus

The pentavalence atom form 4 covalent bonds with the silicon atoms.Since a pentavalence atom has 5 electrons, there is an extra electron left as a free electron.Each pentavalent in the silicon crystal produces one free electron.Therefore, the pentavalence atom is called a donor.

The more pentavalence impurity is added, the more free electron is produced, hence the greater conductivity of the semiconductor.Electrons outnumber the holes, thus this semiconductor is called an n-type semiconductor. N stands for negative.

P-type semiconductor• Add trivalent atoms into a semiconductor.• Trivalent atoms – atoms with 3 valence electrons.• Examples:

• Aluminium• Boron• Gallium

Trivalent atom forms 4 covalent bonds with the silicon atoms.A hole exists in the valence orbit of each trivalent atom.It is called an acceptor atom.The more trivalent impurity is added, the more holes are created in the semiconductor, hence the greater the conductivity of the semiconductor.The holes outnumber the free electrons, thus this semiconductor is called a p-type semiconductor. P stands for positive.

Compare n-type and p-type semiconductor

Semiconductor diodes• Simplest semiconductor device.• Made by joining a p-type and n-type semiconductor.• Allows current to flow in only ONE DIRECTION.

p-n junction• The boundary between the p-type and n-type region.• At the p-n junction, electrons from the n-side move to the

p-side and recombine with the holes.• Holes from the p-side move to the n-side and recombine

with the electrons.• As a result of this flow, the n-side has a net positive

charge, and the p-side has a net negative charge.

Depletion layer• The region around the junction is left with neither holes

nor free electrons.• This neutral region has no charged carriers.• Poor conductor of electricity.

Forward bias• The p-type diode is

connected to the +ve terminal and the n-type is connected to the –ve terminal of a battery.

• The diode conducts current because the holes from p-type material and electrons from n-type material are able to cross over the junction.

• The light bulb lights up.

Reverse bias• The n-type is connected to

the +ve terminal and the p-type is connected to the –ve terminal of the battery.

• The reversed polarity causes a very small current to flow as both electrons and holes are pulled away from the junction.

• When the potential difference due to the widen depletion region equals the voltage of the battery, the current will cease.

• The light bulb does not light up.

Diodes as rectifiers• Converts alternating current (AC) to direct current (DC).• Rectification: A process of converting an AC into a DC by

using a diode.• Two types:

• Half-wave rectification• Full-wave rectification

Half-wave rectification

Full-wave rectification

Smoothing

4.3 TRANSISTOR

Transistor• A transistor is a double p-n junctions with three terminals:

• emitter (e), • base (b) • collector (c).

• The emitter emits charged carriers through the thin base layer to be collected by the collector.

• Two types of transistor:• Npn transistor• Pnp transistor

The arrow on the emitter shows the direction of current flow.

• In npn transistor, the emitter sends NEGATIVE electrons to the collector.

• In pnp transistor, the emitter sends POSITIVE holes to the collector.

• The current in the collector lead is called collector current, Ic.

• The base current, IB, is used to control the collector current through the transistor. The base current can be used to switch the collector current on & off.

Collector circuit is controlled by the case circuit.Current will only flow in the collector circuit when the base circuit is switched on.

How to connect a transistor• Transistor should always

be connected in such a way that:• Emitter base circuit is always

forward bias• Collector base circuit is

always reverse bias

• Example:

• Example

• Example:

• IC >>>>>>>> IB

• Unit for IB is μA ; unit for IC is mA.

•

Transistor as an AUTOMATIC SWITCH:If there is no current flow in the case circuit, there is also no current flow in the collector circuit.If IB=0, then IC=0, transistor is switch off.If IB flows, then IC flows, transistor is switch on.

Transistor as a CURRENT AMPLIFIER:A small change in the base current results in a big change in the collector current.

• Example of transistor as AMPLIFIER.

• Example of transistor as AUTOMATIC SWITCH

Light (LED), heat (thermistor)or sound (microphone).

• Voltage across the base can be controlled by a potential divider.

• According to the potential divider rule, the voltage across resistor R1 and R2 can be calculated by the formula:

Thus, by varying the resistance of R1 and R2, we can control the voltage across the base V2, and turn the light bulb on and off.

LDR – Light dependent resistor• A resistor sensitive to light.• VERY HIGH resistance in darkness and a VERY LOW

resistance in light.

• In a Light Operating Switch, we connect and LDR to a potential divider.

• The voltage across the base vary according to the intensity of light in the surrounding.

Example 1• .

Example 2• .

4.4 LOGIC GATE

Logic Gates• A circuit that has one or more input signals but only

produces ONE OUTPUT SIGNAL.• Each input and output can be either high (logic 1) or low

(logic 0).• 0 represents 0 Voltage, and 1 represents a non-zero

voltage.• Function in daily lives: Security lamps, alarm systems,

washing machines.• Switching on and off are controlled by electronic switched

made up of logic gates.

Types of logic gates • AND gate• OR gate• NOT gate• NAND gate• NOR gate

AND gate• For input to be ON, input A & B must be ON.

OR gate• For output to be ON, at least one input must be ON.

NOT gate• Output in ON when the input is OFF; and vice versa.

NAND gate• It is same as an AND gate with its output inverted by a

NOT gate.

NOR gate• It is the same as an OR gate with its output inverted by a

NOT gate.

Combination of logic gates• Example 1

• Example 2

• Example 3

• Example 4

Formula list